This invention relates to a semiconductor device comprising a semiconductor body having a metallized contact area soldered to a metallic member with a solder which comprises at least 80% lead, the balance of the solder being substantially all indium and silver. The invention also relates to a solder for use in such devices.
The use of a solder for bonding a semiconductor body to a metallic member is common in the semiconductor industry, particularly in the manufacture of non-hermetic power semiconductor devices where it is important that the bond can withstand mechanical and thermal stress as well as corrosion. Thus, the solder itself must be capable of providing a strong bond, as well as having the ability to resist rupture and corrosion due to either environmental influences or fatigue resulting from cyclic thermal stress.
To be more specific, the solder should have good wetting characteristics relative to the surfaces to be joined in order to achieve optimum surface contact for strong bonding.
In the case of power semiconductor rectifiers it is usual for the contact areas of the semiconductor body in which the device is formed to be metallised with a system of two or more different metal layers, the outermost being a thin layer of gold typically 0.45 micrometers thick. Because of their compatibility with silicon and their good wettability, solders consisting of lead and tin have been widely used for bonding metallic contact members to power silicon rectifiers. Unfortunately, however solders containing tin exhibit the phenomenon of gold scavenging. That is to say, the tin tends to dissolve the gold, removing it from the metallized contact areas. Thus the contact areas can become denuded of their gold metallization. Moreover, the gold can react with the tin and form hard intermetallics which cause brittleness of the bond when the solder cools. Having scavenged the gold, the tin can also alloy with the semiconductor material of the rectifier, thus impairing it both electrically and mechanically.
In their article entitled "Proforma on Preforms" which appeared on pages 45 to 48 of the September 1975 issue of Solid State Technology, C. E. T. White and H. C. Sohl mention an alternative solder for use with silicon power rectifiers. This solder, which consists of 92.5% lead, 5% indium and 2.5% silver is said to have a better resistance to thermal cycling than the alloy of lead (95%) and tin (5%). Unfortunately this lead, indium and silver alloy is described as having a poorer wettability than it's lead-tin counterpart.
Nevertheless, the lead-indium-silver alloy has become widely used in the manufacture of power semiconductor devices and slight variants are available from different solder manufacturers. Thus, for example Semi-Alloys Inc. Of Mount Vernon, N.Y., U.S.A., in their 1980 catalog offer alloys with the following ratios of lead, indium and silver respectively, namely 93%, 5%, 2% and 90%, 5%, 5%.
The relatively wide acceptance of these lead-indium-silver alloys reflects the fact that the industry is resigned to accept compromise viz. reduced gold scavenging at the expense of reduced wetting characteristics. Nevertheless, there is a need for a solder which combines high resistance to thermal cycling with improved wetting characteristics.
It is known that the indium and silver constitutents of the alloy go some way to improving the wettability of the solder. On the face of it, therefore, it might be reasonable to conclude that improved wetting would result from increasing the overall content of the indium and silver and conversely, that a reduction in the content of either of these constituents would impair the wetting action of the solder.